1. Field of the invention
The present invention relates to an analog/digital converting semiconductor integrated circuit, and more specifically to an improved layout of an input protection circuit for an analog/digital (A/D) converter in the form of an integrated circuit composed of complementary metal-oxide-semiconductor (MOS) transistors formed on a semiconductor substrate.
2. Description of related art
Recently, A/D converters which are composed of complementary MOS transistors formed on a semiconductor substrate have been improved to realize a high speed operation and a high precision. On the other hand, the A/D converters have been often used in a noisy environment, for example for control of engines of automobiles. In many cases, the A/D converters has a plurality of selectable analog input channels. In the case that the A/D converter having a plurality of selectable analog input channels is used in the noisy environment, noise will intrude from a non-selected analog input channel into a selected analog input channel, with the result that the precision of the A/D conversion will remarkable decrease.
This adverse influence to the A/D conversion precision due the noise will become further remarkable if the A/D convertor has a higher conversion precision. At present, it is estimated that the A/D conversion precision will be made further higher in the future, and therefore, the adverse influence of noise will become more significant. In the A/D converter, accordingly, it is an important current problem awaiting to be solved, to effectively suppress the influence of a noise intruded from a non-selected channel.